What Causes Severe Stomach Pain And Vomiting?

Hello. My daughter (6 yr old) has been complaining of stomachaches for about a week now. She threw up once 2 days ago. She says it feels like pinching. She hasn t been eating a lot. Couple bites here n there. I thought maybe she was constipated but her stomach is not hard at all. She drinks a lot of water. She has dark circles under her eyes. Just wondering what might be causing the pain? Any help will do. Thank you